When a mouse tumor is grown in a host that can mount an immune reaction against it the dose of X-rays required for cure may be reduced by as much as half. The interplay of X-rays and immunity in killing cells is being studied in the C3H mouse ascites tumor BP8, which can with some difficulty be made to elicit immunity in syngeneic C3H mice. This immunity is usually not strong enough to secure rejection of implanted viable cells but it can be detected using tumor cells which have been previously labeled with the thymidine analog 125-I-labeled iododeoxyuridine (IUdR). Loss of radioactivity is monitored by whole-body gamma counting and this loss can be equated with loss of cells which results from either immune reactions or other therapeutic measures. The kinetics of tumor-cell-killing by weak immunity and by X-rays and by stronger immunity in allogeneic mice undergoing primary or secondary responses following immunization against the tumor has been studied. Methods of potentiation of the weak immunity to the tumor specific immunity in syngeneic mice are being investigated along with studies on the evolution of actively-induced immunity.